Optical fiber and optical fiber coupler, erbium-doped optical fiber amplifier, and optical waveguide using the same

ABSTRACT

An optical fiber that includes a core containing a first concentration of germanium, an inner cladding arranged on the core, the inner cladding containing a second concentration of germanium and having a first diffusion coefficient, and an outer cladding arranged on the inner cladding, the outer cladding having a second diffusion coefficient, where the first diffusion coefficient is larger than the second diffusion coefficient, and where the first concentration of germanium is about 200% or more of the second concentration of germanium. An optical fiber constructed in this manner can be spliced with an optical fiber having a different MFD, such as a single-mode optical fiber or an erbium-doped optical fiber, with low splice loss and a sufficient splicing strength.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2002-199959, filed on Jul. 9, 2002 inthe Japanese Patent Office, the entire content of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The invention relates to an optical fiber that is employed in an opticalcomponent used for optical communications, to an optical fiber coupler,to an erbium-doped optical fiber amplifier, and to an optical waveguideusing the same. More specifically, the invention relates to an opticalfiber that can couple with another optical fiber having a different modefield diameter (MFD) with low splice loss and a sufficient splicingstrength when fusion-spliced together, to an optical fiber coupler, toan erbium-doped optical fiber amplifier, and to an optical waveguideusing the same.

DESCRIPTION OF RELATED ART

When two optical fibers having different mode field diameters(hereinafter referred to as “MFDs”) are fusion-spliced together, spliceloss occurs. A method for reducing such splice loss is known in whichoptical fibers are preheated before they are fusion-spliced, or opticalfibers are subjected to a heat treatment by means of additionalelectrical discharge, for example, after they are fusion-splicedtogether, so that a dopant contained in the core diffuses into thecladding and the MFD of the optical fiber having a smaller MFD isenlarged, which results in reduction in the difference of the MFDs ofthe optical fibers.

The change in the core radius due to diffusion of a dopant contained ina core of an optical fiber is expressed by the following Formula (1):r ₂=(r ₁ ²+4Dt)^(1/2)   (1)

In the above-described Formula (1), r₁ is a core radius before thediffusion of the dopant and r₂ is a core radius after the diffusion ofthe dopant. D is a diffusion coefficient and “t” is the length of timefor which the optical fibers are heated. Since the normalized frequencyof an optical fiber (∝ core radius×Δ^(1/2)) remains constant even whenthe dopant in the core diffuses, the MFD is proportional to the coreradius r₂ in the above-described Formula (1). Thus, as the core radiusr₂ is enlarged according to the above-described Formula (1) by causingthe dopant in the core to diffuse, the MFD enlarges accordingly.

Upon fusion-splicing optical fibers having different MFDs together, inorder to reduce the splice loss by application of heat treatment (e.g.,by additional electrical discharge after fusion splicing), severalrequirements should be met. As such requirements, the speed ofenlargement of the MFD of each of optical fibers to be spliced togetherwith respect to heating time must be different, i.e., diffusioncoefficients of the dopants must be different. In addition, the speed ofenlargement of the MFD of an optical fiber having a smaller MFD withrespect to heating time must be larger, i.e., DLM<DSM (where DLM is thediffusion coefficient of the optical fiber having a larger MFD and DSMis the diffusion coefficient of the optical fiber having a smaller MFD).

If these requirements are met, the speed of enlargement of the MFD ofthe optical fiber having a smaller MFD will become greater than thespeed of enlargement of the MFD of the optical fiber having a larger MFDwhen a dopant contained in the core is diffused by application of heat(e.g., by means of additional electrical discharge). As a result, thedifference in the MFDs will be reduced with respect to heating time, andthe splice loss is reduced.

One example of well-known methods for splicing optical fibers havingdifferent MFDs together is a technique disclosed in Japanese Patent No.2911932. In this method, upon fusion-splicing: (1) a single-mode opticalfiber which has zero-dispersion wavelength around 1.3 μm (hereinafterreferred to as “single-mode optical fiber”) that is used as an opticaltransmission path for optical communications; and (2) an optical fiberhaving a numerical aperture between 0.24 and 0.15 (hereinafter referredto as “high numerical aperture optical fiber”) together, splice loss canbe reduced by heat treatment during the fusion splicing and post-heattreatment after fusion splicing. The single-mode optical fiber has acore diameter of about 8 μm, an MFD of about 10 μm, and a refractiveindex difference Δn between the core and the cladding of about 0.004.

In contrast, the high numerical aperture optical fiber has a smallercore diameter (about 4 μm) and MFD (about 4 μm) than the single-modeoptical fiber, and has a relatively large refractive index difference Δnbetween the core and the cladding of between 0.02 and 0.008. Therefore,the concentration of a dopant in the core of the high numerical apertureoptical fiber is higher than the concentration of a dopant in the coreof the single-mode optical fiber. Since the concentration of a dopant inthe core is higher in the high numerical aperture optical fiber, thecore thereof has a lower softening temperature and consequently thediffusion rate of the dopant at a given temperature is significantlygreater compared to the single-mode optical fiber. As a result, thediffusion coefficient of the high numerical aperture optical fiber ishigher than the diffusion coefficient of the single-mode optical fiber.The above-described requirements for reducing the splice loss aresatisfied because the high numerical aperture optical fiber has asmaller MFD and a larger diffusion coefficient while the single-modeoptical fiber has a larger MFD and smaller diffusion coefficient.

In contrast, when optical fibers that have different MFDs and similardiffusion coefficients are fusion-spliced together, it is difficult torealize low splice loss using the splicing method mentioned above. Amethod for reducing splice loss for such cases is known in which acladding of an optical fiber having a smaller MFD is doped with fluorinesuch that the diffusion rate of the dopant in the core of this opticalfiber becomes higher than the diffusion rate of a dopant in a core of anoptical fiber having a larger MFD. One example of this technique is amethod for fusion-splicing an erbium-doped optical fiber employed in anerbium-doped optical fiber amplifier and a dispersion-shiftedsingle-mode optical fiber together.

In this method, the MFD of the erbium-doped optical fiber is 5 μm andthe MFD of the dispersion-shifted single-mode optical fiber is 8 μm at asignal wavelength of 1550 nm, for example. The cladding of theerbium-doped optical fiber is doped with fluorine such that thediffusion rate of a dopant contained in the core into the cladding isincreased. Although the dopant in the core diffuses to the cladding andthe MFD is enlarged by the application of heat after fusion-splicing theerbium-doped optical fiber and the dispersion-shifted single-modeoptical fiber together, the splice loss is reduced to 0.05 dB or smallersince the speed of enlargement of the MFD of the erbium-doped opticalfiber that has a smaller MFD is greater than the speed of enlargement ofthe MFD of the dispersion-shifted single-mode optical fiber.

Examples of fusion-splicing optical fibers having different MFDstogether include connecting between a fused and elongated optical fibercoupler employed in an erbium-doped optical fiber amplifier and anerbium-doped optical fiber or a single-mode optical fiber.

FIG. 8 is a schematic diagram illustrating one example of theconstitution of an erbium-doped optical fiber amplifier.

The erbium-doped optical fiber amplifier of this example generallyincludes an erbium-doped optical fiber 1, a 980 nm semiconductor laser 2that is an excitation light source for exciting the erbium-doped opticalfiber 1, an optical fiber coupler 3 for multiplexing the excitationlight and the signal light, a single-mode optical fiber 4 for connectingthese components, and a 980 nm cut-off shifted single-mode optical fiber5.

In the erbium-doped optical fiber amplifier of this example, theerbium-doped optical fiber 1 and the optical fiber coupler 3 are opticalcomponents that include an optical fiber.

The erbium-doped optical fiber 1 and the optical fiber coupler 3 arefusion-spliced, and the optical fiber coupler 3 and the single-modeoptical fiber 4 are fusion-spliced.

The MFD of the single-mode optical fiber 3 for light in a 1550 nmwavelength band is about 10 μm. The effective cut-off wavelength of thesingle-mode optical fiber 3 is 1300 nm or less.

The effective cut-off wavelength of the erbium-doped optical fiber 1 andthe optical fiber coupler 3 must be smaller than 980 nm since the lightoutput from the 980 nm semiconductor laser 2 is required to bepropagated in a single mode in the erbium-doped optical fiber 1 and theoptical fiber coupler 3.

It is required that the optical fiber coupler 3 propagate light with awavelength of 980 nm in a single mode, and propagate light at awavelength of 1550 nm with low loss. However, since the effectivecut-off wavelength of the optical fiber coupler 3 is located in awavelength region far shorter than 1550 nm, the bending loss tends to berelatively increased when propagating light in a 1550 nm wavelengthband. Accordingly, the optical fiber employed in the optical fibercoupler 3 should have a large relative refractive index differencebetween the core and the cladding, and should have a low bending loss ina 1550 nm wavelength band. Furthermore, to reduce the effectivecut-offwavelength of the optical fiber coupler 3 to 980 nm or lowerwhile increasing the relative refractive index difference between thecore and the cladding of the optical fiber, the core diameter of theoptical fiber included in the optical fiber coupler 3 should be smaller.Consequently, the MFD is reduced accordingly.

On the other hand, the core doped with erbium of the erbium-dopedoptical fiber 1 is required to be excited by excitation light outputfrom the 980 nm semiconductor laser 2 that is an excitation lightsource. Accordingly, the erbium-doped optical fiber 1 generally has ahigh numerical aperture in order to enhance the excitation efficiency.Thus, the MFD is reduced accordingly.

It should be noted that in order to obtain a high-performanceerbium-doped optical fiber amplifier, upon fusion-splicing anerbium-doped optical fiber and an optical fiber coupler, andfusion-splicing an optical fiber coupler and a single-mode opticalfiber, they are required to be spliced with low loss, and the splicedportion is required to have a sufficient strength for practical use.

The size of a MFD for light having a wavelength of 1550 nm is about 10μm in a single-mode optical fiber, about 6.5 μm in an optical fibercoupler, and about 5.5 μm in an erbium-doped optical fiber.

An optical fiber coupler is required to be spliced with either asingle-mode optical fiber or an erbium-doped optical fiber with lowloss, as well as to have a sufficient strength for practical use.Although the MFD of an optical fiber coupler has been conventionallysmall, the relative refractive index difference between the core and thecladding of an optical fiber included in an optical fiber coupler isincreased so as to reduce the bending loss for light in a 1550 nmwavelength band, which results in an increase in the numerical aperture.Accordingly, splice loss between an optical fiber coupler and asingle-mode optical fiber can be reduced.

Here, FIG. 9 is a diagram illustrating the change in the MFDs withrespect to heating time of an optical fiber coupler and a single-modeoptical fiber.

FIG. 9 indicates that a minimum splice loss is attained at a point inwhich the MFDs of the optical fiber coupler and the single-mode opticalfiber are matched. It should be noted that although the above-describedFormula (1) prescribes that the square of the MFD is proportional toheating time, the square of the MFD is not necessarily proportional toheating time since the diffusion coefficient varies depending on theheating temperature.

However, as for the splicing strength, the fusion method disclosed inJapanese Patent No. 2911932 is not necessarily the one that can providea sufficient strength. When an optical fiber coupler and a single-modeoptical fiber are fusion-spliced according to the fusion methoddisclosed in Japanese Patent No. 2911932, a heating time of about 30seconds is required until splice loss reaches the lowest value. Whenoptical fibers ofthe same kind are fusion-spliced with a conventionalfusion splicing method, only 2 to 3 seconds of splicing time issufficient for a fusion splicing the optical fibers with low splice lossand a sufficient strength. A fusion splicing time for fusion-splicing anoptical fiber coupler and a single-mode optical fiber of about 30seconds is considerably longer than the fusion splicing optical fibersof the same kind. Thus, the splicing strength between an optical fibercoupler and a single-mode optical fiber is not necessarily comparable tothe splicing strength between the optical fibers of the same kind.

Furthermore, when splicing an erbium-doped optical fiber and an opticalfiber coupler, it is difficult to achieve low loss and to obtain aspliced portion having a sufficient strength for practical use with themethod disclosed in Japanese Patent No. 2911932 or other methods. Thisis because the values of the MFD of an erbium-doped optical fiber andthe MFD of an optical fiber coupler are relatively close, and both theerbium-doped optical fiber and the optical fiber coupler have highnumerical apertures. Since an erbium-doped optical fiber and an opticalfiber coupler have high numerical apertures, concentrations of thedopant in the core are high and diffusion coefficients are large in theerbium-doped optical fiber and the optical fiber coupler. As a result,the heating time of a spliced portion required until the MFDs oftheerbium-doped optical fiber and the fiber coupler are matched and thelowest loss is achieved is very short.

Here, FIG. 10 is a diagram illustrating the change in the MFD of anoptical fiber coupler and an erbium-doped optical fiber with respect toheating time.

FIG. 10 indicates that the MFDs of the erbium-doped optical fiber andthe fiber coupler are matched and the lowest loss is achieved before thefusion splicing is completed. Thus, if the fusion splicing is stopped atthe point in time when the MFDs is matched, a splicing having asufficient strength for practical use cannot be obtained whereas lowsplice loss is achieved.

Furthermore, in order to enhance the light amplification characteristicof an erbium-doped optical fiber, it is required to dope aluminum, i.e.,one type of dopant, into the core at a high concentration. The doping ofaluminum at a high concentration will cause an increase in the diffusioncoefficient of the dopant. Accordingly, as the light amplificationcharacteristic of the erbium-doped optical fiber is enhanced, it becomesmore difficult to improve splicing strength between the erbium-dopedoptical fiber and the optical fiber coupler. For the reasons mentionedabove, there is a need for an improved optical fiber coupler in order toachieve both an improvement in the amplification characteristic of anerbium-doped optical fiber and an improvement in splicing strengthbetween the erbium-doped optical fiber and the optical fiber coupler.

Upon splicing an erbium-doped optical fiber and an optical fibercoupler, in order to achieve both low loss and sufficient splicingstrength for practical use, the diffusion coefficient of the opticalfiber included in the optical fiber coupler should be increased suchthat the heating time until the MFDs of an erbium-doped optical fiberand an optical fiber coupler are matched becomes sufficiently long. As amethod for increasing the diffusion coefficient of the optical fiberincluded in the optical fiber coupler, a method of doping fluorine intoa cladding that is described in a known document (A. Wada, T. Sakai, D.Tanaka, T. Nozawa and R. Yamauchi, OAA 1991, FD3, 1991, “High-EfficiencyErbium-Doped Fiber Amplifier using Mode Field Diameter AdjustingTechnique”) can be applied to an optical fiber included in the opticalfiber coupler. However, the effect of increasing the diffusioncoefficient will be limited if the amount of doped fluorine is small.

When the amount of doped fluorine is large, the relative refractiveindex difference between the core and the cladding will significantlychange, which results in a change in the transmission characteristics.In addition, splicing with low splice loss cannot be achieved by simplyincreasing the diffusion coefficient of the optical fiber included inthe optical fiber coupler to a value larger than the diffusioncoefficient of an erbium-doped optical fiber since the MFDs of theoptical fiber coupler and the erbium-doped optical fiber will not bematched by a change in the heating time.

BRIEF SUMMARY OF THE INVENTION

The invention was conceived in view of the above-mentioned background,and one object thereof is to provide an optical fiber that can befusion-spliced with another optical fiber having a different mode fielddiameter (MFD) with low splice loss and a sufficient splicing strength,an optical fiber coupler, erbium-doped optical fiber amplifier, and anoptical waveguide using the same.

In one aspect, the invention is an optical fiber, including: a corecontaining a first concentration of germanium; an inner claddingarranged on the core, the inner cladding containing a secondconcentration of germanium and having a first diffusion coefficient; andan outer cladding arranged on the inner cladding, the outer claddinghaving a second diffusion coefficient, where the first diffusioncoefficient is larger than the second diffusion coefficient, and wherethe first concentration of germanium is about 200% or more of the secondconcentration of germanium.

In another aspect of the optical fiber of the invention, the core issilica glass.

In another aspect of the optical fiber of the invention, the core isarranged at the center of the optical fiber.

In another aspect of the optical fiber of the invention, the innercladding is concentric to the core.

In another aspect of the optical fiber of the invention, the innercladding is silica glass containing germanium, phosphorus, and fluorine.

In another aspect of the optical fiber of the invention, the outercladding is silica glass.

In another aspect of the optical fiber of the invention, a diameter ofthe inner cladding is between 10 μm and 40 μm.

In another aspect of the optical fiber of the invention, a concentrationof the germanium is between 0.2 and 1.5% by weight, a concentration ofphosphorus is between 0.5 and 1.5% by weight, and a concentration offluorine is between 0.1 and 1.0% by weight in the inner cladding.

In another aspect of the optical fiber of the invention, an outerdiameter of the optical fiber is between 70 μm and 90 μm.

In another aspect of the optical fiber of the invention, twostress-applying parts are symmetrically disposed with respect to thecore within the cladding.

In other aspects of the invention, the optical fiber mentioned above isprovided in an optical fiber coupler an erbium-doped optical fiberamplifier, or as a pigtail fiber of an optical waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a schematic diagram of an exemplary embodiment of theinvention illustrating a portion of an erbium-doped optical fiberamplifier;

FIG. 2 is a diagram illustrating refractive index profiles of theoptical fiber shown in FIG. 1;

FIG. 3 is a diagram illustrating the change of MFDs with respect toheating time when fusion-splicing an erbium-doped optical fiber with theoptical fiber shown in FIG. 1, or with a conventional high NA opticalfiber;

FIG. 4 is a diagram illustrating the change in the MFDs with respect toheating time of exemplary optical fibers of the invention andcomparative optical fibers;

FIG. 5 is a cross-sectional view of an exemplary embodiment of theinvention showin a PANDA polarization maintaining optical fiber;

FIG. 6 is a schematic diagram of an exemplary embodiment of theinvention showing a tap coupler;

FIG. 7 is a schematic cross-sectional view of an exemplary embodiment ofthe invention showing a package of an erbium-doped optical fiberamplifier;

FIG. 8 is a schematic diagram illustrating one example of theconstitution of an erbium-doped optical fiber amplifier;

FIG. 9 is a diagram illustrating the change in the MFDs with respect toheating time of an optical fiber coupler and a single-mode opticalfiber; and

FIG. 10 is a diagram illustrating the change in the MFDs with respect toheating time of an optical fiber coupler and an erbium-doped opticalfiber.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the invention will now be described below byreference to the attached Figures. The described exemplary embodimentsare intended to assist the understanding of the invention, and are notintended to limit the scope of the invention in any way.

In an exemplary embodiment, the optical fiber of the invention is anoptical fiber including a core that is provided at a center, the corebeing made of silica glass containing at least germanium; an innercladding having a large diffusion coefficient, the inner cladding beingprovided around the core and concentric to the core, and an outercladding having a small diffusion coefficient, the outer cladding beingprovided around the inner cladding, in which the inner cladding containsgermanium, and a concentration of germanium in the core is about 200% ormore of a concentration of germanium in the inner cladding.

By constructing an optical fiber having such a structure, it is possibleto modify the manner in which the MFD of the optical fiber changes withrespect to heating time of the spliced portion after fusion splicing.Specifically, in the optical fiber of the invention, the MFD rapidlyincreases in an early stage of the heating, and the speed of enlargementof the MFD gradually slows down in the middle and end stages. Such aphenomenon is observed because germanium doped in the core diffuses tothe cladding when the heating is started, and the MFD rapidly increases.The speed of migration of germanium is relatively large in an earlystage of the heating since germanium diffuses in the inner cladding inwhich the diffusion coefficient is larger. In the middle and end stagesof the heating, as the germanium reaches the outer cladding in which thediffusion coefficient is smaller, the speed of enlargement of the MFD isdelayed and the rate of the enlargement gradually slows down since thespeed of the diffusion of germanium into the outer cladding slows downs.

For example, when an optical fiber constructed according to theinvention and an erbium-doped optical fiber are fusion-spliced and thespliced portion between these fibers is heated, the MFD ofthe opticalfiber ofthe invention is greater in an early stage of the heating.However, the speed of enlargement of the MFD of the optical fiber of theinvention gradually slows down in the middle and end stages of theheating, and eventually the MFD of the optical fiber of the inventionand the MFD ofthe erbium-doped optical fiber are matched. As a result,the splice loss between the optical fiber of the invention and theerbium-doped optical fiber is reduced. In addition, the spliced portionbetween the optical fiber of the invention and the erbium-doped opticalfiber has a sufficient strength since the spliced portion has beenheated long enough to reduce the splice loss.

Furthermore, since the MFD of the optical fiber of the invention rapidlyenlarges in the early stage of the heating when the optical fiber of theinvention and a single-mode optical fiber are fusion-spliced, it ispossible to match the MFD of the optical fiber of the invention and theMFD of the single-mode optical fiber in a relatively short time. As aresult, it is possible to prevent deterioration of the strength of theoptical fiber caused by long heating since the time required forenlarging the MFD of the optical fiber of the invention can beshortened. Accordingly, fusion splicing having both a sufficientsplicing strength and low loss is realized.

The inner cladding of the optical fiber of the invention may be made ofsilica glass containing germanium (Ge), phosphorus (P), and fluorine(F), and the outer cladding is made from silica (SiO2). It should benoted, however, that silicon tetrachloride (SiCl4) may be used as astarting material for forming the outer cladding, and a small amount ofchlorine is contained as a impurity in the outer cladding in such acase, which does not cause any particular problem. By constructing anoptical fiber having such a structure, the diffusion rate of germaniumdoped in the core into the inner cladding will be relatively increasedwhile the diffusion rate of germanium into the outer cladding will berelatively reduced. By doping three elements, i.e., germanium,phosphorus, and fluorine, in the inner cladding, it is possible toincrease the diffusion rate of the dopants compared to an optical fiberin which only fluorine is doped.

The diameter of the core of the optical fiber of the invention may be 9μm or smaller.

Furthermore, the diameter of the inner cladding of the optical fiber ofthe invention may be between 10 μm and 40 μm, and more preferably, butnot necessarily,between 15 μm and 25 μm.

If the diameter of the inner cladding is smaller than 10 μm, the effectof inhibiting the enlargement of the MFD will occur in an early stage ofthe heating, and the splice loss will reach the lowest value before asufficient splicing strength is obtained when fusion-splicing theoptical fiber of the invention and an erbium-doped optical fiber. Incontrast, if the diameter of the inner cladding exceeds 40 μm, whenfusion-splicing the optical fiber of the invention and a single-modeoptical fiber, the MFD of the optical fiber of the invention and the MFDofthe single-mode optical fiber will be matched before the effect ofinhibiting the enlargement of the MFD is obtained by heating the opticalfiber of the invention. As a result, the splice loss will reach thelowest value before a sufficient splicing strength is achieved.

By setting the diameter of the inner cladding of the optical fiber ofthe invention to the above-mentioned range, the optical fiber of theinvention can be spliced with a single-mode optical fiber or anerbium-doped optical fiber with low loss and a sufficient splicingstrength.

When fabricating an optical fiber coupler, two optical fibers arearranged in parallel, and portions ofthe fibers are fused by heatingthem, and the fused portion is elongated while heating. In this process,the MFDs of the two optical fibers will enlarge since a dopant containedin the core diffuses to cladding due to heating and elongation in theoptical fibers. The rate of the enlargement of the MFD caused by heatingand elongation is related to the coupling length (the length of thefused-elongated portion) of an optical fiber coupler, and it is possibleto reduce the coupling length to a relatively small value if the speedof enlargement of the MFD is large. By reducing coupling length, it ispossible to reduce the size of an optical fiber coupler. It should benoted, however, that if the speed of enlargement of the MFD is toolarge, the MFD of an end of the coupling portion (the Y-shaped portionof the optical fiber coupler) will be enlarged to an excessive degree.As a result, there will be a difference between the MFD in regions otherthan the coupling portion and the MFD of the end of the coupling portionin the optical fiber used in the optical fiber coupler.

If there is a large difference in the MFD in certain regions, a loss isincurred when light propagates through such regions, which results in anexcessive loss in the optical fiber coupler. In order to obtain anexcellent optical fiber coupler with a reduced size and low excess loss,it is required to use an optical fiber in which the speed of enlargementof the MFD is large during heating and elongation while the MFD does notenlarge to an excessive degree when heating and elongation is completed.

According to the optical fiber of the invention, by setting the diameterof the inner cladding to values in the above-mentioned range, it ispossible to obtain an excellent optical fiber coupler with a reducedsize and low excess loss. In addition, the optical fiber coupler can befusion-spliced with a single-mode optical fiber or an erbium-dopedoptical fiber with low loss and a sufficient splicing strength.

In the optical fiber of the invention, the concentration of germanium inthe core may be between 3.0 and 21.0% by weight, and the concentrationof fluorine may be between 0.1 and 1.0% by weight.

Preferably, but not necessarily, the concentration of germanium may bebetween 0.2 and 1.5% by weight, the concentration of phosphorus may bebetween 0.5 and 1.5% by weight, and the concentration of fluorine may bebetween 0.1 and 1.0% by weight in inner cladding. More preferably, butagain not necessarily, the concentration of germanium may be between 0.8and 1.2% by weight, the concentration of phosphorus may be between 0.8and 1.2% by weight, and the concentration of fluorine may be between 0.3and 0.7% by weight in the inner cladding.

The effect of inhibiting the enlargement of the MFD that is achieved byheating the optical fiber of the invention is related to the diffusionrates of dopants, i.e., germanium, phosphorus, and fluorine in the innercladding, and consequently is related to the concentrations of thesedopants in the inner cladding. By setting the concentrations of dopantsin the inner cladding of the optical fiber of the invention to theabove-mentioned ranges, the optical fiber of the invention can bespliced with a single-mode optical fiber or an erbium-doped opticalfiber with low loss and a sufficient splicing strength.

If the concentrations of dopants, i.e., germanium, phosphorus, andfluorine, in the inner cladding are smaller than the lower limitsdescribed above, the diffusion rates of dopants in the inner claddingwill be limited, and the splice loss will reach the lowest value beforea sufficient splicing strength is achieved when fusion-splicing theoptical fiber of the invention and an erbium-doped optical fiber. Incontrast, if the concentrations of dopants, i.e., germanium, phosphorus,and fluorine, in the inner cladding exceeds the upper limits describedabove, the MFD of the optical fiber of the invention and the MFD of asingle-mode optical fiber will be matched before the effect ofinhibiting the enlargement of the MFD is obtained that is achieved byheating the optical fiber of the invention. As a result, the splice losswill reach the lowest value before a sufficient splicing strength isachieved.

In addition, the outer diameter of the optical fiber of the inventionmay be between 70 μm and 90 μm, and more preferably, but notnecessarily, between 75 μm and 85 μm.

Additionally, in optical components employing an optical fiber, such asan erbium-doped optical fiber amplifier, size reduction of thecomponents has been demanded. In order to reduce the size of suchoptical components, it is required to reduce the outer diameter of anoptical fiber below the conventional outer diameter of 125 μm so that anallowable bending radius of the optical fiber is reduced and the fiberis compactly encased. According to the optical fiber of the invention,by setting the outer diameter of the optical fiber to values in theabove-mentioned range, it is possible to obtain an excellent opticalcomponent with a reduced size and a low bending loss. In addition, theoptical component can be fusion-spliced with a single-mode optical fiberor an erbium-doped optical fiber with low loss and a sufficient splicingstrength.

Furthermore, it is possible to reduce the size of a fused and elongatedtype optical fiber coupler by employing the optical fiber of theinvention having an outer diameter between 70 μm and 90 μm. Thereduction in the size of a fused and elongated type optical fibercoupler is achievable by reducing the length of the coupling length (thelength of the fused-elongated portion) of the coupler. Since the opticalfiber of the invention has a smaller outer diameter of between 70 μm and90 μm than a conventional optical fiber having an outer diameter of 125μm, it is possible to reduce the coupling length for obtaining modecoupling to a value smaller than the conventional optical fiber. Thus,it is possible to reduce the size of the optical fiber coupler.

Furthermore, the optical fiber of the invention may be a polarizationmaintaining optical fiber that has two stress-applying parts that aresymmetrically disposed with respect to the core within the claddingaround the core.

Polarization interleave multiplexing is one of the dense wavelengthmultiplexing techniques that meets the recent demands forcommunications. A polarization maintaining erbium-doped optical fiberamplifier can amplify signal light while maintaining the plane ofpolarization, and such an amplifier is essential for the polarizationinterleave multiplexing. Accordingly, an erbium-doped optical fiber, oran optical fiber coupler, or the like that is used as main components ofsuch a polarization maintaining erbium-doped optical fiber amplifiershould exhibit a polarization maintaining characteristic. It is knownthat polarization maintaining optical components can be obtained byfabricating fiber-type optical components using a polarizationmaintaining optical fiber, such as a PANDA optical fiber having twostress-applying parts that are symmetrically disposed with respect tothe core within the cladding around the core or a so-called bow-tieoptical fiber. In addition, in a polarization maintaining erbium-dopedoptical fiber amplifier, a splicing with low loss and a sufficientsplicing strength is required when splicing respective components. Forthe above-described reasons, a polarization maintaining optical fibermay be used as the optical fiber of the invention that is used in apolarization maintaining erbium-doped optical fiber amplifier.

Furthermore, the optical fiber coupler of the invention may be a fusedand elongated type optical fiber coupler fabricated using the opticalfiber of the invention.

Since the optical fiber of the invention can be spliced with asingle-mode optical fiber or an erbium-doped optical fiber with low lossand a sufficient splicing strength, the optical fiber coupler of theinvention will be similarly an optical fiber coupler spliced with lowloss and a sufficient splicing strength.

The erbium-doped optical fiber amplifier of the invention is fabricatedusing the optical fiber of the invention, and generally includesfiber-type optical components and non-fiber-type optical components.

In general, optical components used in an erbium-doped optical fiberamplifier have different MFDs. For this reason, in the erbium-dopedoptical fiber amplifier, it is essential to connect components havingdifferent MFDs with low loss and a sufficient splicing strength. In theerbium-doped optical fiber amplifier of the invention, a splicing withlow loss and a sufficient splicing strength can be realized, and as aresult, the performance can be enhanced by inserting the optical fibersof the invention between connectors of each of the components to connectthem, rather than directly connecting the components.

An optical waveguide of the invention is fabricated using the opticalfiber of the invention as a pigtail fiber, and is a fiber-type opticalcomponent in which two or more different kinds of optical fibers oroptical components are coupled.

In general, it is required to introduce light having a wavelength of1550 nm from a single-mode optical fiber used for optical communicationsinto an optical waveguide. However, the MFD of a single-mode opticalfiber and the MFD of the optical waveguide are different. Accordingly,by using an optical waveguide in which the optical fiber of theinvention is used as a pigtail fiber, this optical waveguide and asingle-mode optical fiber can be spliced with low loss and a sufficientsplicing strength.

The above exemplary embodiment will now be further described in view ofthe following detailed examples and comparisons. It should beunderstood, however, that the invention is not limited to the particularexamples described herein.

EXAMPLE 1

FIG. 1 is a schematic diagram illustrating a portion of an erbium-dopedoptical fiber amplifier.

The erbium-doped optical fiber amplifier of this example generallyincludes an erbium-doped optical fiber 11 and an optical fiber coupler12 that is made by fusing and elongating optical fibers 14 and 14, and asingle-mode optical fiber 13 that is spliced with the optical fiber 14of the invention used in the optical fiber coupler 12.

As shown in FIG. 1, the optical fiber coupler 12, and the erbium-dopedoptical fiber 11 and the single-mode optical fiber 13 are splicedtogether via fusion-spliced portions 16 in the erbium-doped opticalfiber amplifier of this example. The splice loss and the splicingstrength of the optical fiber coupler 12 are the same as the splice lossand the splicing strength of the optical fibers 14 and 14 used therein.The optical fiber of the invention and a conventional high NA opticalfiber were used as the optical fibers 14 and 14 employed in the opticalfiber coupler 12, and an optical fiber coupler 12 employing two opticalfibers of the invention and another optical fiber coupler 12 employingtwo conventional optical fibers having a high numerical aperture werefabricated.

Here, FIG. 2 is a diagram illustrating refractive index profiles ofoptical fibers, in which FIG. 2(a) illustrates a refractive indexprofile of the optical fiber of the invention, and FIG. 2(b) illustratesa refractive index profile of the conventional high NA optical fiber.

As shown in FIG. 2(a), the optical fiber of the invention has atwo-layered cladding structure in which the outer periphery of the coreis surrounded by cladding in which the refractive index is smaller thanin the core, and the cladding includes an inner cladding and an outercladding, each of which has a different composition.

The core was made of silica-based glass doped with germanium (Ge) andfluorine (F) (SiO₂/GeO₂/F), and the contents of the dopants were 12% byweight, and 0.2% by weight for germanium and fluorine, respectively.

The inner cladding was made of silica-based glass doped with germanium,phosphorus (P), and fluorine (SiO₂/GeO₂/P₂O₅/F), and the contents of thedopants were about 1% by weight, about 1% by weight, and about 0.5% byweight for germanium, phosphorus and fluorine, respectively.

The outer cladding is made of silica-based glass (SiO₂).

There was little relative refractive index difference Δ between theinner cladding and the outer cladding, and the diameter of the innercladding was 20 μm.

As shown in FIG. 2(b), the conventional high NA optical fiber is anoptical fiber in which the outer periphery of the core is surrounded bythe cladding in which the refractive index is smaller than in the core.

The core was made of silica-based glass doped with germanium andfluorine (SiO₂/GeO₂/F), and the contents of the dopants were 12% byweight, and 0.2% by weight for germanium and fluorine, respectively.

The cladding was made of pure silica-based glass (SiO₂).

Characteristics of each of the optical fibers used in the erbium-dopedoptical fiber amplifier shown in FIG. 1 are listed in Table 1. TABLE 1MFD Effective Relative RI Core (@1550 cut-off difference diameter nm)wavelength Type of fiber Δ (%) NA (μm) (μm) (μm) Optical fiber 1.0 0.213.5 6.5 0.92 of the invention Conventional 1.0 0.21 3.5 6.5 0.92 high NAfiber Single-mode 0.35 0.12 8.2 10 1.28 optical fiber Er-doped 1.22 0.232.9 5.5 0.88 optical fiber

In both the optical fiber of the invention and the conventional high NAoptical fiber, the relative refractive index difference between the coreand the cladding Δ was 1.0%, the core diameter was 3.5 μm, the numericalaperture was 0.21, the effective cut-off wavelength was 0.92 μm, and theMFD with respect to light having a wavelength of 1550 nm was 6.5 μm.

To each of optical fiber couplers 12 that were made using either one ofthe two optical fibers, the erbium-doped optical fiber 11 and thesingle-mode optical fiber 13 were fusion-spliced, and the splice lossand the splicing strength of the fusion-spliced portion 16 weremeasured.

Procedures to measure the splice loss and the splicing strength of thefusion-spliced portion 16 will be explained.

After ends of each of the optical fibers to be spliced were flattenedusing a fiber cleaver or the like, the optical fibers were set to aconventional arc discharge-type fusion splicer. Then, the optical fiberswere fusion-spliced in the presence of the arc, and the arc wasmaintained to conduct the heat treatment to further enlarge the MFDs.The arc current during the heat treatment was selected such that thetemperature of the optical fibers was increased up to the glasssoftening point around between 1400 and 1600° C. During the heattreatment while fusion splicing and after splicing, the change in thepropagation loss of the fusion-spliced portions 16 was measured todetermine the minimum splice loss of the fusion-spliced portions 16 inwhich the propagation loss reached the lowest value. The change inpropagation loss with respect to the length of time for which the arcwas maintained was determined. The time to the minimum splice loss wasdefined as the arc maintaining time until the splice loss reached thelowest value. Then, 50 optical fibers were fusion-spliced using the timeto the minimum splice loss for various combination of fibers, andtension test was carried out for the fusion-spliced portions 16. Uponconducting the tension test, the fusion-spliced samples were secured toa well-known tension tester, and the samples were pulled at a tensionspeed in which extension strain in one minute becomes 5% and tensionforce when the samples broke was recorded. The test was conducted forthe 50 optical fibers, and the failure tension with which the cumulativefailure probability becomes 50% was calculated. Here, the cumulativefailure probability is defmed as a probability of breakage when acertain failure tension or lower is applied. In this example, failuretension data of 50 optical fibers from the tension test were analyzedaccording to the method of Weibull analysis to determine therelationship between the cumulative failure probability and the failuretension. The results are listed in Table 2. TABLE 2 Failure tension(GPa) when Min. cumulative splice Time to failure Fiber to be Fiber tobe loss min. splice probability spliced (1) spliced (2) (dB) loss (sec.)becomes 50% Single-mode Optical fiber of 0.1 10 2.4 optical fiber theinvention Single-mode Conventional 0.2 30 2.0 optical fiber high NAfiber Er-doped Optical fiber of 0.1 2 2.5 optical fiber the inventionEr-doped Conventional 0.2 1 1.2 optical fiber high NA fiber

The results listed in Table 2 indicate that the minimum splice loss whenspliced with a single-mode optical fiber was 0.1 dB for the opticalfiber of the invention is, and 0.2 dB for the conventional high NAoptical fiber. The time to the minimum splice loss was 10 seconds forthe optical fiber of the invention, and 30 seconds for the conventionalhigh NA optical fiber. This is because the MFD of the optical fiber ofthe invention and the MFD of the single-mode optical fiber are matchedin a relatively short time since the diffusion rate of the dopants ofthe core is faster in the optical fiber of the invention.

In the tension test, the failure tension at which the cumulative failureprobability becomes 50% was 2.4 GPa when the optical fiber of theinvention and a single-mode optical fiber were fusion-spliced, and was2.0 GPa when the conventional high NA optical fiber and the single-modeoptical fiber were fusion-spliced. The conventional high NA opticalfiber in which the time to the minimum splice loss was longer wasweaker. In addition, when comparing strength of a spliced portionbetween single-mode optical fibers that are considered to have arelatively large splicing strength, the failure tension of the splicedportion between the single-mode optical fibers were 2.5 GPa, which wasalmost the same as the splicing strength between the optical fiber ofthe invention and a single-mode optical fiber. Thus, it was confirmedthat the optical fiber of the invention can be spliced with asingle-mode optical fiber with low loss and a sufficient strength.

In contrast, the minimum splice loss when spliced with an erbium-dopedoptical fiber was 0.1 dB for the optical fiber of the invention is, andwas 0.2 dB for the conventional high NA optical fiber.

FIG. 3 is a diagram illustrating the change of the MFDs with respect toheating time when fusion-splicing an erbium-doped optical fiber and theoptical fiber of the invention or a conventional high NA optical fiber.The time to the minimum splice loss was 2 seconds for the optical fiberof the invention, and was 1 second for the conventional high NA opticalfiber. Since the diffusion rate of the dopant in the core of theerbium-doped optical fiber is sufficiently larger than that of theconventional high NA optical fiber, the MFDs of the fibers are matchedin a short time. In contrast, since the diffusion rate of the dopant inthe core of the optical fiber of the invention is larger than thediffusion rate of the dopant in the core of the conventional high NAoptical fiber in the early stage of the heating, it is possible tolengthen the time until the MFD of the optical fiber of the invention ismatched with the MFD of the erbium-doped optical fiber longer than theconventional high NA optical fiber. As a result, it is possible tolengthen the time to the minimum splice loss to a relatively largevalue.

In the tension test, the failure tension at which the cumulative failureprobability becomes 50% was 2.5 GPa when the optical fiber of theinvention and an erbium-doped optical fiber were fusion-spliced, whichwas almost the same as the splicing strength of a spliced portionbetween single-mode optical fibers that is considered to have arelatively splicing strength. In contrast, the failure tension whenfusion-splicing the conventional high NA optical fiber and theerbium-doped optical fiber was 1.2 GPa, meaning that spliced portion wasvery weak. This is because the sufficient splicing strength cannot beobtained since the time to the minimum splice loss in the combination ofthe conventional high NA optical fiber and the erbium-doped opticalfiber is very short. For this reason, although the splicing between theconventional high NA optical fiber and the erbium-doped optical fiberhas a low splice loss, the splicing cannot withstand practical use dueto a weak splicing strength. In contrast, the optical fiber of theinvention could be spliced with the erbium-doped optical fiber with lowsplice loss and a sufficient splicing strength.

EXAMPLE 2

In another exemplary comparison, three types of optical fibers having aprofile shown in FIG. 2(a) were provided, except that each of which hasa different inner cladding diameter. The diameters of the inner claddingof the optical fibers were 10 μm, 20 μm, or 40 μm, respectively.

As shown in FIG. 2(a), the optical fiber of the invention has atwo-layered cladding structure in which the outer periphery of the coreis surrounded by the cladding in which the refractive index is smallerthan in the core, and the cladding includes an inner cladding and anouter cladding, each of which has a different composition.

The core was made of silica-based glass doped with germanium (Ge) andfluorine (F) (SiO₂/GeO₂/F), and the contents of the dopants were 12% byweight, and 0.2% by weight for germanium and fluorine, respectively.

The inner cladding was made of silica-based glass doped with germanium,phosphorus (P), and fluorine (SiO₂/GeO₂/P₂O₅/F), and the contents of thedopants were about 1% by weight, about 1% by weight, and about 0.5% byweight for germanium, phosphorus and fluorine, respectively.

The outer cladding is made of silica-based glass (SiO₂).

There was little relative refractive index difference Δ between theinner cladding and the outer cladding.

All ofthe optical fibers were fabricated to have the same core structureas that of the optical fiber in Example 1 above. The relative refractiveindex difference A of the core was 1.0%, the numerical aperture was0.21, the core diameter was 3.5 m, the MFD (1550 nm) was 6.5 μm, and theeffective cut-off wavelength was 0.92 μm.

Similar to Example 1, each of the optical fibers having different innercladding diameters was fusion-spliced with a single-mode optical fiberor an erbium-doped optical fiber, and the splice loss and the splicingstrength were measured. The measurement was carried out using the sameprocedures as Example 1. The results are listed in Table 3. TABLE 3Failure tension (GPa) when Min. cumulative splice Time to failure Fiberof the Fiber to be loss min. splice probability invention spliced (dB)loss (sec.) becomes 50% Inner cladding of Single-mode 0.12 18 2.2 10 μmoptical fiber Inner cladding of Single-mode 0.10 10 2.4 20 μm opticalfiber Inner cladding of Single-mode 0.12 4.5 2.5 40 μm optical fiberInner cladding of Er-doped 0.12 1.8 2.3 10 μm optical fiber Innercladding of Er-doped 0.10 2.0 2.5 20 μm optical fiber Inner cladding ofEr-doped 0.12 4.0 2.4 40 μm optical fiber

The results listed in Table 3 indicate that although the optical fiberhaving an inner cladding diameter of 20 μm exhibited the most favorablesplice loss, the difference between the optical fibers having an innercladding diameter of 10 μm or 40 μm was about 0.02 dB. All ofthe threeoptical fibers exhibited an excellent splicing strength. Thus, it wasconfirmed that the optical fiber ofthe invention having a diameter ofthe inner cladding between 10 μm and 40 μm can be spliced with asingle-mode optical fiber or an erbium-doped optical fiber with low lossand a sufficient strength.

COMPARATIVE EXAMPLE 1

As a matter of further comparison, two types of optical fibers havingthe same profile as optical fibers in Example 2 were provided, exceptthat the inner cladding diameters were different from the optical fibersin Example 2. The diameters of the inner cladding of the optical fiberswere 5 μm or 45 μm, respectively.

Similar to Example 1, each of the optical fibers having different innercladding diameters was fusion-spliced with a single-mode optical fiberor an erbium-doped optical fiber, and the splice loss and the splicingstrength were measured. The measurement was carried out using the sameprocedures as Example 1. The results are listed in Table 4. TABLE 4Failure tension (GPa) when Min. cumulative splice Time to failure Fiberof the Fiber to be loss min. splice probability invention spliced (dB)loss (sec.) becomes 50% Inner cladding of Single-mode 0.18 25 2.1 5 μmoptical fiber Inner cladding of Single-mode 0.18 1.5 2.0 45 μm opticalfiber Inner cladding of Er-doped 0.15 1.2 1.8 5 μm optical fiber Innercladding of Er-doped 0.15 5.0 2.4 45 μm optical fiber

The results listed in Table 4 indicate that some optical fibersexhibited higher splice loss than the optical fibers in Example 2 andhad a splicing strength below 2.0 GPa.

Thus, it was confirmed that an optical fiber having an inner claddingdiameter out of the range between 10 μm and 40 μm exhibited inferiorsplicing characteristic to the optical fiber of the invention whenspliced with a single-mode optical fiber or an erbium-doped opticalfiber.

EXAMPLE 3

In a further comparative example, three types of optical fibers A, B,and C having a profile shown in FIG. 2(a) were provided, each having adifferent dopant content.

The inner cladding of the optical fiber A was made of silica-based glassdoped with germanium, phosphorus (P), and fluorine (SiO₂/GeO₂/P₂O₅/F),and the contents of the dopants were 0.2% by weight, 0.5% by weight, and0.1% by weight for germanium, phosphorus and fluorine, respectively.

The inner cladding of the optical fiber B was made of silica-based glassdoped with germanium, phosphorus (P), and fluorine (SiO₂/GeO₂/P₂O₅/F),and the contents of the dopants were about 1% by weight, about 1% byweight, and about 0.5% byweight for germanium, phosphorus and fluorine,respectively.

The inner cladding of the optical fiber C was made of silica-based glassdoped with germanium, phosphorus (P), and fluorine (SiO₂/GeO₂/P₂O₅/F),and the contents of the dopants were 1.5% by weight, 1.5% by weight, and1.0% by weight for germanium, phosphorus and fluorine, respectively.

Parameters other than the dopant concentration in the inner claddingwere the same as those of the optical fibers in Example 1.

In other words, the core was made of silica-based glass doped withgermanium (Ge) and fluorine (F) (SiO₂/GeO₂/F), and the contents of thedopants were 12% by weight, and 0.2% by weight for germanium andfluorine, respectively.

The outer cladding is made of silica-based glass (SiO₂).

There was little relative refractive index difference Δ between theinner cladding and the outer cladding.

All ofthe structural parameters of the core were the same as thestructural parameters of the optical fiber of the invention listed inTable 1. The relative refractive index difference Δ of the core was1.0%, the numerical aperture was 0.21, the core diameter was 3.5 μm, theMFD (@1550 nm) was 6.5 μm, and the effective cut-off wavelength was 0.92μm. The diameter of the inner cladding was 20 μm.

Similar to Example 1, each of the optical fibers having different dopantcontents in the inner cladding was fusion-spliced with a single-modeoptical fiber or an erbium-doped optical fiber, and the splice loss andthe splicing strength were measured. The measurement was carried outusing the same procedures as Example 1. The results are listed in Table5. TABLE 5 Failure tension (GPa) when Min. cumulative splice Time tofailure Fiber of the Fiber to be loss min. splice probability inventionspliced (dB) loss (sec.) becomes 50% Fiber A Single-mode 0.11 16 2.3(low conc.) optical fiber Fiber B Single-mode 0.10 10 2.4 (middle conc.)optical fiber Fiber C Single-mode 0.12 5.0 2.4 (high conc.) opticalfiber Fiber A Er-doped 0.12 1.7 2.1 (low conc.) optical fiber Fiber BEr-doped 0.10 2.0 2.5 (middle conc.) optical fiber Fiber C Er-doped 0.122.5 2.5 (high conc.) optical fiber

The results listed in Table 5 indicate although the optical fiber Bexhibited the most favorable splice loss, the difference between theoptical fiber A or C was about 0.02 dB. All three fibers had anexcellent splicing strength.

Thus, it was confirmed that the optical fiber of the invention in whichthe concentration of germanium is between 3.0 and 21.0% by weight in thecore, and the concentration of fluorine is between 0 and 1.0% by weightin the core, the concentration of germanium is between 0.2 and 1.5% byweight in the inner cladding, and the concentration of phosphorus isbetween 0.5 and 1.5% by weight in the inner cladding, and theconcentration of fluorine is between 0.1 and 1.0% by weight in the innercladding can be spliced with a single-mode optical fiber or anerbium-doped optical fiber with low loss and a sufficient strength.

COMPARATIVE EXAMPLE 2

As a matter of comparison, two types of optical fibers D and E havingthe same profile as optical fibers in Example 3 were provided, exceptthat the dopant concentrations of the inner cladding were different fromthe optical fibers in Example 3.

The inner cladding of the optical fiber D was made of silica-based glassdoped with phosphorus (P) and fluorine (SiO₂/P₂O₅/F), and the contentsof the dopants were 0.2% by weight and 0.05% by weight for phosphorusand fluorine, respectively.

The inner cladding of the optical fiber E was made of silica-based glassdoped with germanium, phosphorus (P), and fluorine (SiO₂/GeO₂/P₂O₅/F),and the contents of the dopants were about 2.0% by weight, about 2.0% byweight, and about 1.2% by weight for germanium, phosphorus and fluorine,respectively.

Similar to Example 1, each of the optical fibers having different dopantcontents in the inner cladding was fusion-spliced with a single-modeoptical fiber or an erbium-doped optical fiber, and the splice loss andthe splicing strength were measured. The measurement was carried outusing the same procedures as Example 1. The results are listed in Table6. TABLE 6 Failure tension (GPa) when Min. cumulative splice Time tofailure Fiber to be Fiber to be loss min. splice probability spliced (1)spliced (2) (dB) loss (sec.) becomes 50% Fiber D Single-mode 0.14 20 2.2(lowest conc.) optical fiber Fiber E Single-mode 0.22 1.5 1.9 (highestconc.) optical fiber Fiber D Er-doped 0.14 1.5 1.9 (lowest conc.)optical fiber Fiber E Er-doped 0.15 3.5 2.4 (highest conc.) opticalfiber

The results listed in Table 6 indicate that some optical fibersexhibited higher splice loss than the optical fibers in Example 3 andhad a splicing strength below 2.0 GPa.

Thus, it was confirmed that an optical fiber in which the concentrationof germanium is 12% by weight in the core, and the concentration offluorine is 0.2% by weight in the core, and the concentration ofgermanium is out of the range between 0.2 and 1.5% by weight in theinner cladding, the concentration of phosphorus is out of the rangebetween 0.5 and 1.5% by weight in the inner cladding, and theconcentration of fluorine is out of the range between 0.1 and 1.0% byweight in the inner cladding exhibited inferior splicing characteristicswhen spliced with a single-mode optical fiber or an erbium-doped opticalcompared to the optical fiber of the invention.

Furthermore, the change in the mode field diameter (MFD) with respect toheating time of the optical fibers A to E mentioned above is shown inFIG. 4.

It was confirmed from FIG. 4 that the optical fibers A, B, and C whichare the optical fibers of the invention, can be fusion-spliced withsingle-mode optical fiber or an erbium-doped optical fiber with low lossand a sufficient strength.

EXAMPLE 4

In another example according to the invention, three types of PANDApolarization maintaining optical fibers having a profile shown in FIG. 5were provided, except that each of which has a different inner claddingdiameter.

The PANDA polarization maintaining optical fibers were optical fibers,each of which includes a core 21 with a diameter of 3.5 μm, an innercladding 22, an outer cladding 23 having a diameter of 125 μm, and twostress-applying parts 24 and 24 that are symmetrically disposed withrespect to the core 21.

The diameters of the inner cladding 22 of the optical fibers were 10 μm,20 μm, or 40 μm, respectively.

The core 21 was made of silica-based glass doped with germanium (Ge) andfluorine (F) (SiO₂/GeO₂/F), and the contents of the dopants were 12% byweight, and 0.2% by weight for germanium and fluorine, respectively.

The inner cladding 22 was made of silica-based glass doped withgermanium, phosphorus (P), and fluorine (SiO₂/GeO₂/P₂O₅/F), and thecontents of the dopants were about 1% by weight, about 1% by weight, andabout 0.5% by weight for germanium, phosphorus and fluorine,respectively.

The outer cladding 23 is made of silica-based glass (SiO₂).

Two stress-applying parts 24 and 24 were made of silica-based glassdoped with boron (SiO₂/B₂O₃).

There was little relative refractive index difference Δ between theinner cladding 22 and the outer cladding 23.

The relative refractive index difference Δ of the core was 1.0%, the MFD(1550 nm) was 6.5 μm, and the effective cut-off wavelength was 0.92 μm.

Similar to Example 1, each of the PANDA polarization maintaining opticalfibers having different inner cladding diameters was fusion-spliced witha single-mode optical fiber or an erbium-doped optical fiber, and thesplice loss and the splicing strength were measured. The measurement wascarried out using the same procedures as Example 1. The results arelisted in Table 7. TABLE 7 Failure tension (GPa) when Min. cumulativePANDA splice Time to failure polarization Fiber to be loss min. spliceprobability maintaining fiber spliced (dB) loss (sec.) becomes 50% Innercladding of Single-mode 0.12 18 2.2 10 μm optical fiber Inner claddingof Single-mode 0.10 10 2.4 20 μm optical fiber Inner cladding ofSingle-mode 0.12 4.5 2.5 40 μm optical fiber Inner cladding of Er-doped0.12 1.8 2.3 10 μm optical fiber Inner cladding of Er-doped 0.10 2.0 2.520 μm optical fiber Inner cladding of Er-doped 0.12 4.0 2.4 40 μmoptical fiber

The results listed in Table 7 indicate that although the optical fiberhaving a diameter of the inner cladding 22 of 20 μm exhibited the mostfavorable splice loss, the difference between the optical fibers havinga diameter of the inner cladding 22 of 10 μm or 40 μm was about 0.02 dB.All of the three optical fibers exhibited an excellent splicingstrength. Thus, it was confirmed that the PANDA polarization maintainingoptical fiber having a diameter of the inner cladding between 10 μm and40 μm can be spliced with a single-mode optical fiber or an erbium-dopedoptical fiber with low loss and a sufficient strength.

CPMPARATIVE EXAMPLE 3

As a matter of comparison, two types of optical fibers having the sameprofile as the PANDA optical fibers in Example 4 were provided, exceptthat the inner cladding diameters were varied from the optical fibers inExample 4. The diameters of the inner cladding of the optical fiberswere 5 μm or 45 μm, respectively.

Similar to Example 1, each of the PANDA polarization maintaining opticalfibers having different inner cladding diameters was fusion-spliced witha single-mode optical fiber or an erbium-doped optical fiber, and thesplice loss and the splicing strength were measured. The measurement wascarried out using the same procedures as Example 1. The results arelisted in Table 8. TABLE 8 Failure tension (GPa) when Min. cumulativePANDA splice Time to failure polarization Fiber to be loss min. spliceprobability maintaining fiber spliced (dB) loss (sec.) becomes 50% Innercladding of Single-mode 0.18 25 2.1 5 μm optical fiber Inner cladding ofSingle-mode 0.18 1.5 2.0 45 μm optical fiber Inner cladding of Er-doped0.15 1.2 1.8 5 μm optical fiber Inner cladding of Er-doped 0.15 5.0 2.445 μm optical fiber

The results listed in Table 8 indicate that some optical fibersexhibited higher splice loss than the optical fibers in Example 4 andhad a splicing strength below 2.0 GPa.

Thus, it was confirmed that a PANDA polarization maintaining opticalfiber having an inner cladding diameter out of the range between 10 μmand 40 μm exhibited inferior splicing characteristic to the PANDApolarization maintaining optical fiber of the invention when splicedwith a single-mode optical fiber or an erbium-doped optical fiber.

EXAMPLE 5

According to another example of the invention, three types of PANDApolarization maintaining optical fibers F, G and H having a profileshown in FIG. 5 were provided, each of which has a different dopantcontent.

The inner cladding 22 of the optical fiber F was made of silica-basedglass doped with germanium, phosphorus (P), and fluorine(SiO₂/GeO₂/P₂O₅/F), and the contents of the dopants were 0.2% by weight,0.5% by weight, and 0.1% by weight for germanium, phosphorus andfluorine, respectively.

The inner cladding 22 of the optical fiber G was made of silica-basedglass doped with germanium, phosphorus (P), and fluorine(SiO₂/GeO₂/P₂O₅/F), and the contents of the dopants were about 1% byweight, about 1% by weight, and about 0.5% byweight for germanium,phosphorus and fluorine, respectively.

The inner cladding 22 of the optical fiber H was made of silica-basedglass doped with germanium, phosphorus (P), and fluorine(SiO₂/GeO₂/P₂O₅/F), and the contents of the dopants were 1.5% by weight,1.5% by weight, and 1.0% by weight for germanium, phosphorus andfluorine, respectively.

Parameters other than the dopant concentration in the inner cladding 22and the presence of the stress-applying parts 24 and 24 were the same asthe optical fibers in Example 1.

In other words, the core was made of silica-based glass doped withgermanium (Ge) and fluorine (F) (SiO₂/GeO₂/F), and the contents of thedopants were 12% by weight, and 0.2% by weight for germanium andfluorine, respectively.

The outer cladding 23 is made of silica-based glass (SiO₂).

There was little relative refractive index difference Δ between theinner cladding 22 and the outer cladding 23.

The relative refractive index difference Δ of the core 21 was 1.0%, theMFD (1550 nm) was 6.5 μm, and the effective cut-off wavelength was 0.92μm. The diameter of the inner cladding 22 was 20 μm.

Similar to Example 1, each of the PANDA polarization maintaining opticalfibers having different dopant contents in the inner cladding wasfusion-spliced with a single-mode optical fiber or an erbium-dopedoptical fiber, and the splice loss and the splicing strength weremeasured. The measurement was carried out using the same procedures asExample 1. The results are listed in Table 9. TABLE 9 Failure tension(GPa) when Min. cumulative PANDA splice Time to failure polarizationFiber to be loss min. splice probability maintaining fiber spliced (dB)loss (sec.) becomes 50% Fiber F Single-mode 0.11 16 2.3 (low conc.)optical fiber Fiber G Single-mode 0.10 10 2.4 (middle conc.) opticalfiber Fiber H Single-mode 0.12 5.0 2.4 (high conc.) optical fiber FiberF Er-doped 0.12 1.7 2.1 (low conc.) optical fiber Fiber G Er-doped 0.102.0 2.5 (middle conc.) optical fiber Fiber H Er-doped 0.12 2.5 2.5 (highconc.) optical fiber

The results listed in Table 9 indicate although the optical fiber Gexhibited the most favorable splice loss, the difference between theoptical fiber F or H was about 0.02 dB. All three fibers had anexcellent splicing strength.

Thus, it was confirmed that the PANDA polarization maintaining opticalfiber of the invention in which the concentration of germanium isbetween 3.0 and 21.0% by weight in the core 21, and the concentration offluorine is between 0 and 1.0% by weight in the core 21, theconcentration of germanium is between 0.2 and 1.5% by weight in theinner cladding, the concentration of phosphorus is between 0.5 and 1.5%by weight in the inner cladding, and the concentration of fluorine isbetween 0.1 and 1.0% by weight in the inner cladding can be spliced witha single-mode optical fiber or an erbium-doped optical fiber with lowloss and a sufficient strength.

COMPARATIVE EXAMPLE 4

As a matter of comparison, two types of PANDA polarization maintainingoptical fibers I and J having the same profile as optical fibers inExample 5 were provided, except that the dopant concentrations of theinner cladding 22 were varied from the optical fibers in Example 3.

The inner cladding of the optical fiber I was made of silica-based glassdoped with phosphorus (P) and fluorine (SiO₂/P₂O₅/F), and the contentsof the dopants were 0.2% by weight, and 0.05% by weight for phosphorusand fluorine, respectively.

The inner cladding of the optical fiber J was made of silica-based glassdoped with germanium, phosphorus (P), and fluorine (SiO₂/GeO₂/P₂O₅/F),and the contents of the dopants were about 2.0% by weight, about 2.0% byweight and about 1.2% by weight for germanium, phosphorus and fluorine,respectively.

Similar to Example 1, each of the PANDA polarization maintaining opticalfibers having different dopant contents in the inner cladding 22 wasfusion-spliced with a single-mode optical fiber or an erbium-dopedoptical fiber, and the splice loss and the splicing strength weremeasured. The measurement was carried out using the same procedures asExample 1. The results are listed in Table 10. TABLE 10 Failure tension(GPa) when Min. cumulative PANDA splice Time to failure polarizationFiber to be loss min. splice probability maintaining fiber spliced (2)(dB) loss (sec.) becomes 50% Fiber I Single-mode 0.14 20 2.2 (lowestconc.) optical fiber Fiber J Single-mode 0.22 1.5 1.9 (highest conc.)optical fiber Fiber I Er-doped 0.14 1.5 1.9 (lowest conc.) optical fiberFiber J Er-doped 0.15 3.5 2.4 (highest conc.) optical fiber

The results listed in Table 10 indicate that some optical fibersexhibited higher splice loss than the optical fibers in Example 5 andhad a splicing strength below 2.0 GPa.

Thus, it was confirmed that a PANDA polarization maintaining opticalfiber in which the concentration of germanium 12% by weight in the core21 and the concentration of fluorine is 0.2% by weight in the core 21,and the concentration of germanium is out of the range between 0.2 and1.5% by weight in the inner cladding 22, the concentration of phosphorusis out of the range between 0.5 and 1.5% by weight in the inner cladding22, and the concentration of fluorine is out of the range between 0.1and 1.0% by weight in the inner cladding 22 exhibited inferior splicingcharacteristics when spliced with a single-mode optical fiber or anerbium-doped optical compared to the optical fiber ofthe invention.

EXAMPLE 6

In another example of the invention, an optical power coupling/dividingcoupler having the structure shown in FIG. 6 was fabricated.

This optical power coupling/dividing coupler is one type of opticalfiber coupler, i.e., a so-called “tap coupler,” which is for extractinga very small amount of light for monitoring lines. This tap coupler wasdesigned such that one percent of the power of signal light having awavelength of 1550 nm is extracted.

In this optical power coupling/dividing coupler, when signal lighthaving a wavelength of 1550 nm is incident to a first port 31, onepercent of the optical power that is divided in the optical fibercoupler 35 is emitted from a third port 33 and the rest of the opticalpower is emitted from a second port 32.

This tap coupler was fabricated using an optical fiber of the invention.The outer diameter of this optical fiber was 80 μm, which is smallerthan the outer diameter of 125 μm of a conventional optical fiber. Thisoptical fiber was an optical fiber that includes a core having adiameter of about 7.2 μm, an inner cladding having a diameter of 20 μm,and an outer cladding having a diameter of 80 μm.

The core was made of silica-based glass doped with germanium (Ge) andfluorine (F) (SiO₂/GeO₂/F), and the contents of the dopants were 6.3% byweight and 0.2% by weight for germanium and fluorine, respectively.

The inner cladding was made of silica-based glass doped with germanium,phosphorus (P), and fluorine (SiO₂/GeO₂/P₂O₅/F), and the contents of thedopants were about 1% by weight, about 1% by weight, and about 0.5% byweight for germanium, phosphorus and fluorine, respectively.

The outer cladding is made of silica-based glass (SiO₂).

The relative refractive index difference Δ of the core was 0.54%, theMFD (@1550 nm) was 8.3 μm, and the effective cut-off wavelength was 1.34μm.

The inner cladding and the outer cladding had almost the same refractiveindex.

Two such optical fibers were provided, arranged in parallel, and fusedand elongated to fabricate a tap coupler that extracts one percent ofthe power of signal light having a wavelength of 1550 nm.

The coupling length of this tap coupler was 5 mm. Since the opticalfiber of the invention included the inner cladding and had an outerdiameter or 80 μm, the coupling length could be reduced. As a result,the size of the tap coupler could be reduced.

COMPARATIVE EXAMPLE 5

As a matter of comparison, atap coupler similar to the tap coupler inExample 6 was fabricated using a conventional optical fiber. The opticalfiber employed in this comparative example was an optical fiber that hasan outer diameter of 125 μm, and includes a core having a diameter ofabout 7.2 μm and a cladding having a diameter of 125 μm.

The core was made of silica-based glass doped with germanium (Ge) andfluorine (F) (SiO₂/GeO₂/F), and the contents of the dopants were 6.3% byweight and 0.2% by weight for germanium and fluorine, respectively.

The cladding was made of pure silica-based glass (SiO₂).

The relative refractive index difference Δ of the core was 0.54%, theMFD (1550 nm) was 8.3 μm, and the effective cut-off wavelength was 1.34μm.

Two such optical fibers were provided, arranged in parallel, and fusedand elongated to fabricate a tap coupler that extracts one percent ofthe power of signal light having a wavelength of 1550 nm.

The coupling length of this tap coupler was 14 mm. It was found that thecoupling length was large and the size of the tap coupler using theconventional optical fiber could not be reduced sufficiently compared tothe tap coupler of Example 6.

EXAMPLE 7

In another example according to the invention, an erbium-doped opticalfiber amplifier 40 having the structure shown in FIG. 7 was fabricated.

This erbium-doped optical fiber amplifier 40 includes a wavelengthdivision multiplexing coupler (hereinafter abbreviated as “WDM coupler”)41 and other components 42 that are contained in a housing 43. Thedimensions of the erbium-doped optical fiber amplifier 40 were height of70 mm by width of 90 mm by depth of 12 mm.

Other parts 42 included an excitation 980-nm laser diode for excitation,an optical isolator, a tap coupler, and the like. In order to house anumber of components, the excess length of the optical fiber of the WDMcoupler 41 was wound about cylindrical members 44, each of which has aradius of 10 mm to save space.

The WDM coupler was fabricated using an optical fiber of the invention.The outer diameter of this optical fiber 45 was 80 μm, which is smallerthan the outer diameter of 125 μm of a conventional optical fiber. Theoptical fiber 45 is an optical fiber that includes a core having adiameter of about 3.1 μm, an inner cladding having a diameter of 20 μm,and an outer cladding having a diameter of 80 μm.

The core was made of silica-based glass doped with germanium (Ge) andfluorine (F) (SiO₂/GeO₂/F), and the contents of the dopants were 14% byweight and 0.2% by weight for germanium and fluorine, respectively.

The inner cladding was made of silica-based glass doped with germanium,phosphorus (P), and fluorine (SiO₂/GeO₂/P₂O₅/F), and the contents of thedopants were about 1% by weight, about 1% by weight, and about 0.5% byweight for germanium, phosphorus and fluorine, respectively.

The outer cladding is made of silica-based glass (SiO₂).

The relative refractive index difference Δ of the core was 1.25%, theMFD (1550 nm) was 6.0 μm, and the effective cut-off wavelength was 0.92μm.

The inner cladding and the outer cladding had almost the same refractiveindex.

The bending loss of the optical fiber 45 when wound with a bendingradius of 10 mm by 5 turns was 0.05 dB in a wavelength of 1.610 μm, andwas less than 0.05 dB in wavelengths shorter than 1.610 μm, which wasalmost 0 dB.

Two of the optical fibers 45 were provided, arranged in parallel, andfused and elongated to fabricate the WDM coupler 41 thatmultiplexes/demultiplexes an excitation light having a wavelength of 980nm and signal light having a wavelength of 1550 nm.

Since the WDM coupler 41 fabricated in the manner described aboveincludes the optical fiber 45 having an outer diameter of 80 μm, itexhibits less strain due to bending than a WDM coupler including aconventional optical fiber having an outer a diameter of 125 μm. Thus,the WDM coupler of the invention that employs the optical fiber 45having an outer diameter of 80 μm exhibits a lower breaking failurerate.

The method for calculating breaking failure rate of optical fibers isdisclosed in Y. Mitsunaga et al., “Strength assurance of optical fiberbased on screening test,” Trans. IEICE July 1983, vol. J.66-B, No. 7,pp. 829-836, 1983. In general, it is required to assure a breakingfailure rate of optical fibers of 10-5 (one failure per 100,000 units)for a duration of 20 years. When an optical fiber having an outer adiameter of 125 μm is employed in a conventional WDM coupler with abending radius of 10 mm, the breaking failure rate would exceed 10-5 byfar in 20 years, meaning that this WDM couple cannot withstand practicaluse. In contrast, it was confirmed from the document described abovethat when an optical fiber of the invention is employed in the WDMcoupler with a bending radius of 10 mm, a breaking failure rate of 10-5can be assured for a duration of 20 years.

Furthermore, the WDM coupler including the optical fiber of theinvention has a coupling length of 5.4 mm, which is far shorter than thecoupling length of a conventional WDM coupler.

Similar to Example 6, since the optical fiber of the invention includesan inner cladding, and has an outer diameter of 80 μm, it is possible toreduce the coupling length. In the WDM coupler 41 of Example 7, thecoupling length could be significantly reduced to 5.4 mm, andconsequently the overall size of the WDM coupler 41 was reduced.

Since the WDM coupler is used in an erbium-doped optical fiberamplifier, the WDM coupler 41 of this example will be spliced to anoptical fiber having a different MFD. Similar to Example 1, since theWDM coupler 41 included an optical fiber in which the concentrations ofthe dopants in the inner cladding were about 1% by weight, about 1% byweight, and about 0.5% by weight, for germanium, phosphorus, andfluorine, the WDM coupler 41 could provide a splicing with low loss anda sufficient strength as Example 1.

Accordingly, it was confirmed that a WDM coupler that includes anoptical fiber of the invention including an inner cladding and having anouter diameter of 80 μm has a smaller dimension and smaller allowablebending radius than a conventional WDM coupler, and exhibits anexcellent splicing characteristic. Accordingly, the WDM coupler of theinvention can be applied to an erbium-doped optical fiber amplifierhaving a reduced size than a conventional erbium-doped optical fiberamplifier.

While the invention has been particularly shown and described withreference to exemplary embodiments thereof, the invention is not limitedto these embodiments. It will be understood by those of ordinary skillin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention as defimedby the following claims.

INDUSTRIAL APPLICABILITY

As described above, since the optical fiber of the invention is anoptical fiber including a core that is provided at a center, the corebeing made of silica glass containing at least germanium, an innercladding having a large diffusion coefficient, the inner cladding beingprovided around the core and concentric to the core, and an outercladding having a small diffusion coefficient, the outer cladding beingprovided around the inner cladding, in which the inner cladding containsgermanium, and a concentration of germanium in the core is about 200% ormore of a concentration of germanium in the inner cladding, it can bespliced with an optical fiber having a different MFD, such as asingle-mode optical fiber or an erbium-doped optical fiber, with lowsplice loss and a sufficient splicing strength.

1. An optical fiber, comprising: a core that is provided at a center,the core being made of silica glass containing at least germanium; aninner cladding made of silica glass, the inner cladding being providedconcentric to the core; and an outer cladding made of silica glass, theouter cladding being provided around the inner cladding, wherein theinner cladding contains germanium, and a concentration of germanium inthe core is about 200% or more of a concentration of germanium in theinner cladding, a diameter of the inner cladding is between 10 μm and 40μm, the inner cladding containing germanium of between 0.2% by weightand 1.5% by weight, phosphorus of between 0.5% by weight and 1.5% byweight, and fluorine of between 0.1% by weight and 1.0% by weight, adiffusion coefficient of germanium diffused in the inner cladding beingrelatively large, a refractive index of the inner cladding beingsubstantially the same as a refractive index of silica glass, adiffusion coefficient of germanium diffused in the outer cladding isrelatively smaller than in the inner cladding, and an enlarged modefield diameter of the optical fiber during splicing with a second silicaglass optical fiber by means of heating using an arc discharge for atime to a minimum splicing loss is matched with an enlarged mode fielddiameter of the second silica glass optical fiber, a failure tensionwith which a cumulative failure probability becomes 50% is over 2.0 GPaafter the fusion splicing.
 2. The optical fiber according to claim 1,wherein fluorine is doped in the core.
 3. The optical fiber according toclaim 1, wherein an outer diameter of the optical fiber is between 70 μmand 90 μm.
 4. The optical fiber according to claim 1, further comprisingtwo stress-applying parts that are symmetrically disposed with respectto the core within the cladding around the core.
 5. The optical fiberaccording to claim 1, wherein the second silica glass optical fiber is asingle mode optical fiber.
 6. The optical fiber according to claim 1,wherein the second silica glass optical fiber is an erbium-doped opticalfiber.
 7. A optical fiber coupler comprising the optical fiber accordingto claim 1.